1. Field of the Invention
The present invention relates to a color image forming apparatus for superimposing images formed on a plurality of photoconductive elements onto a transfer member. More particularly, the present invention relates to a color image forming apparatus that prevents color image deviation by forming a color detecting pattern on a transfer member.
2. Background
Conventionally, in a digital color image forming apparatus having a plurality of photoconductive elements, color images are respectively written on each of the photoconductive elements and then superimposed. Therefore, a positional deviation of the color images tends to occur due to curvature of an image scanning line, inclination of the image scanning line, and magnification error, from variations in the accuracy of the mechanical position of each of the photoconductive elements, in the positional accuracy of image writing onto each of the photoconductive elements, in the quality of the writing lens itself, and from fluctuations in writing position and magnification, or the like, of the entire digital color image forming apparatus caused by a speed error of the photoconductive elements and temperature variations thereof. Consequently, these phenomena appear as a color unevenness and/or a color positional deviation, and are factors that degrade image quality.
A method for obtaining an image in which the above-mentioned deviations are corrected is disclosed, for example, in Japanese Laid-Open Patent Application No. 278074/1988, and Japanese Laid-Open Patent Application No. 279270/1988. In these Applications, there is proposed a method in which reflection-type sensors are mounted on each photoconductive element and deviation is detected by detecting a time lag between the detection time of each of the sensors. Another method, for example, disclosed in Japanese Laid-Open Patent Application No. 286864/1988, forms a pattern with a horizontal line and a slanting line on a transfer member and measures the arrival time of each of the lines to a reflection-type sensor on each photoconductor and the magnification in a main scanning direction, and then corrects a clock and a write starting position. Still another method, for example, disclosed in Japanese Laid-Open Patent Application No. 286866/1988 and Japanese Laid-Open Patent Application No. 279273/1988, converts output data from the reflection-type sensor of each photoconductive element into binary data, and by calculating a central value, the effect of variations in the width of a line, which causes variations in the detecting time of the sensor, is reduced.
However, the aforementioned method of detecting positional deviation sometimes produces errors because it utilizes a transfer belt that may have a roughness such as a scar thereon. Therefore, as a solution to the above problem, a method has been proposed in which a positional deviation detecting pattern is compared with a surface state such as a scar or the like on the transfer belt, by initially reading the surface state and then storing related information in a memory, as disclosed in Japanese Laid-Open Patent Application No. 337754/1992.
However, the aforementioned conventional method requires a large-capacity memory because the surface state, such as a scar or the like on the transfer belt, must be initially read and the related information must be stored in the memory. Also, calculations are complex because the surface state must be compared with the positional deviation detecting pattern. Consequently, the above conventional method has the problem of increased manufacturing costs.